In many applications, the qualities of lightweight construction and high resistance to fragmentation and corrosion damage are highly desirable characteristics for a pressure vessel. These design criteria have been met for many years by the development of high pressure composite (fiber reinforced resin matrix) containers; for instance, container shells fabricated of laminated layers of wound fiberglass filaments or various types of other synthetic filaments which are bonded together by a thermal-setting or thermoplastic resin. An elastomeric or other non-metal resilient liner or bladder often is disposed within the composite shell to seal the vessel and prevent internal fluids from contacting the composite material.
Such composite vessels have become commonly used for containing a variety of fluids under pressure, such as storing helium, natural gas, nitrogen, rocket or other fuel, propane, etc. The composite construction of the vessels provides numerous advantages such as lightness in weight and resistance to corrosion, fatigue and catastrophic failure. These attributes are due to the high specific strengths of the reinforcing fibers or filaments which typically are oriented in the direction of the principal forces in the construction of the pressure vessels.
Filament wound vessels often are constructed in a spherical shape or an elongated cylindrical shape with generally hemispherical or hemispheroidal ends for use in high pressure applications. At least one of the ends has an opening, and a boss is positioned in the opening, with the boss reliably joining the inner liner with the outer composite shell such that fluid is prevented from penetrating between the liner and the shell. In many applications, such as in the aerospace industry, composite pressure vessels may be required to contain extremely high pressures, operating at 25,000 p.s.i. with design burst values in the range of 50,000 p.s.i. Consequently, as internal pressure increases, the interface of the boss, the liner and the outer shell is subjected to extreme structural loading.
Examples of pressure vessels of the character described above, including boss-liner attachment systems, are shown in copending application Ser. No. 902,725, dated Jun. 23, 1992 and assigned to the assignee of the present invention; as well as in U.S. Pat. No. 5,253,779 to Sirosh, dated Oct. 19, 1993. While both of these items of prior art may be successful for their intended purposes of compensating for varying stress generated between the boss and the composite shell of a pressure vessel, shearing stress between the boss and the inner liner, and steep strain gradients through the shell, problems still are encountered in these types of pressure vessels, particularly when the vessels are considerably elongated between their ends.
More particularly, an elongated pressure vessel generally includes a cylindrical side wall configuration including the outer composite shell and the inner liner, with at least one end being generally hemispherical (or hemispheroidal) in configuration. Such a vessel may be as long as 300 inches, such as on a semi-trailer truck for carrying natural gas, or other appropriate applications. The design specifications for such a vessel include accommodating temperatures as low as -40.degree. F. When the pressure vessel cools to such temperatures, the inner liner tends to shrink, but the outer composite shell does not shrink as much. This difference between the coefficient of thermal contraction/expansion between the inner liner and the outer shell often causes the liner to separate from the boss and to separate and develop gaps between the liner and the shell at the hemispherical ends of the vessel. In fact, in a vessel which is 100 inches long and which is subjected to a temperature of -40.degree. F., the inner liner may shrink a full one inch more than the outer shell. The shrinkage in the cylindrical areas of the liner literally pulls on the hemispherical ends of the liner. This invention is directed to solving those problems and preventing separation of the liner in the ends of an elongated pressure vessel.